In the testing of circuit boards, and particularly printed circuit boards, it is necessary to simultaneously contact numerous points on the circuit boards with electrically conductive probe-pins, mounted collectively on a probe plate, and make satisfactory electrical contact with the electrical components on the circuit boards. This has customarily been accomplished in the past by using a vacuum to depress the circuit board into a "bed of nails"-type array of electrical probes. Once all necessary electrical contacts are effected, separate electrical circuitry carries out the analysis to determine if the board is operating correctly.
Another method of engaging the electrical contacting probe-pins in a testing fixture is to apply simultaneously opposed arrays of probes to both sides of a board under test. Relatively balanced forces must be applied to the board from opposed sides to ensure there is no undue deflection from the pressure of the contacting probes. If necessary, dummy probes are used to ensure that a balance of forces is obtained.
For every specific circuit board design, it is necessary to have a corresponding probe plate that carries precisely placed probe-pins that are aligned with predetermined contact points on components that are mounted on the circuit board. In high volume testing situations it may be necessary to change probe plates on short notice and with a minimal interruption of the flow of circuit boards being tested.
The fixture should also be capable of receiving circuit boards with a minimum of delay, handling and adjustment, while at the same time providing accurate and precise placement of the board within the fixture.
One test fixture of existing in the prior art provides for the testing of circuit boards between pairs of vertical probe plates which are pneumatically closed on the subject board from both sides. Inward displacement of the probe plates is effected by pressure plates mounted on linear bearings, which, in their activating linkages and power cylinder, constitute a pressure frame within the fixture.
In the prior art, the probe plates are installed individually in the fixture during the set-up process. This procedure produces complications discussed further, below. In the operation of the prior art fixture, a board is placed in a protruding receiving frame which serves as a carrier to facilitate insertion of the board into the machine. Once the board is placed within the carrier, the carrier is drawn into the machine by a pneumatic cylinder.
In all machines of this class it is essential to make provision to ensure ultimately that tooling or index pins on the probe plates are aligned with index holes formed within the board. The index holes in a circuit board are precisely located holes from which components on the circuit board are precisely located. Similarly, the index pins on the probe plates (and their counterpart receiving holes on the opposing probe plates) are precisely located with respect to the probe-pins on the probe plates. If all of these parts are properly aligned, the probe pins will contact all of the components correctly during the testing process.
In the prior art different probe plates (to accommodate different circuit boards) must be individually mounted, and aligned, within the main frame of the test fixture. This is customarily done while the test fixture is located on the shop floor, along the production/testing line. The probe plates themselves must be precisely aligned with each other. Since, in the prior art, the probe plates are interconnected through their individual pressure plates and interconnecting bearings within the fixture, all of these components forming an exterior loop must be precisely manufactured to exacting tolerances.
The high precision required to ensure the precise placement of the probe plates imposes significant demands on the machining accuracy of the components involved. Every component interposed between the probe plates contributes towards an accumulation of variances due to individual tolerances. The number of components used in the prior art system described imposes costly manufacturing tolerance standards on each element and contributes to unreliability.
In the prior art the procedure for mounting the probe plates individually in the main frame of the test fixture, while the testing fixture is on the testing line, disrupts production.